Sinus or migraine - how to tell

I am a regular on the IBS forum but would like to ask a question of you good people here. I have always been prone to headaches and, because they come over and behind the eyes and eyebrows as well as in the maxillary areas, thought they were sinus ones. My nose is constantly blocked and the neti pot treatment can't penetrate from one nostril to the other. However, last night's offering was one of the worst headaches I have ever had. It was accompanied by feelings of nausea and tremors. How can you tell whether a headache is sinus related or migraine? I have brief periods of retinal migraine and wonder whether the head pain is in fact more due to a migraine cause than sinus.

I have heard that keeping up water intake can be beneficial in warding off or ameliorating the effects of headache. What are your experiences?

Many thanks.